1. Field of the Invention:
This invention relates to a copying apparatus, and more particularly to a copying apparatus in which an original is scanned by an optical system to produce a copy image on a light receiving sheet and the copy image is transferred onto an image receiving sheet.
2. Description of the Prior Art:
An image forming method using a light receiving sheet is disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 58(1983)-88,739. In this method, a combination of a light receiving sheet coated with photosensitive microcapsules containing non-colored dye and a photohardening material in a rupturable capsule and an image receiving sheet coated with a developer material which colors the non-colored dye is employed. The light reflected from an image is projected onto the light receiving sheet to form a selectively hardened pattern thereon, then the image receiving sheet is placed over the light receiving sheet. Thereafter, the combination is applied with pressure to rupture the non-hardened microcapsules, thereby coloring the non-colored dye to develop a copy image on the image receiving sheet.
When a copy image is formed on an image receiving sheet by a copying apparatus using the above-mentioned method, the density of the copy image depends on the luminous intensity of the light source of the optical system, the scanning speed of the optical system, and the variation in the sensitivity of the light receiving sheet. In such a copying apparatus, the luminous intensity of the light source or the scanning speed of the optical system must be adjusted in order to obtain a copy image of the desired density. This may be accomplished by controlling a power source for the light source to adjust the luminous intensity of the light source, or by controlling the driving motor for the optical system to adjust the scanning speed, in accordance with the density level which has been set by an operator.
When the density of a copy image is adjusted only by increasing or decreasing the luminous intensity of the light source, the voltage applied to the lamp of the light source must be changed over an extremely wide range to cover all possible density levels of copy images which may be set by an operator, causing the change in the color temperature of the lamp. The above will raise a problem in that the spectral wavelength distribution changes so that the hue of a image of a color copy is different from that of the original image.
On the other hand, the copy magnification ratio is altered by the change of the scanning speed. When a copy apparatus is designed so as to adjust the density of a copy image only by changing the scanning speed, therefore, the control of the scanning speed must cover a very wide range of scanning speeds so that both the adjustment of the image density and that of the copy magnification ratio are conducted simultaneously, which causes a substantially large difference between the minimum and maximum scanning speeds. This causes another problem to arise in that it is very difficult to control the driving motor of the optical system.
The sensitivity of a light receiving sheet depends upon the sensitivity of the photosensitive microcapsules which are scattered over a substantially wide range. When the sensitivity of the photosensitive microcapsules used in a light receiving sheet is low, hence, the apparatus is controlled so as to increase the luminous intensity of the light source. In contrast, when the sensitivity of the photosensitive microcapsules is high, the apparatus is controlled so as to decrease the luminous intensity of the light source. Namely, in a prior art copying apparatus, the unevenness of the sensitivity among light receiving sheets of different kinds is compensated by adjusting the luminous intensity of the light source. Therefore, the scanning speed in a prior art copying apparatus must be low, even when a light receiving sheet coated by photosensitive microcapsules of a high sensitivity is used, causing a prolonged period of time for the copying operation.